With the new energy vehicles’ rapid rising, fast charging and fast discharging of power battery is gradually becoming the mainstream working mode. The heat transfer characteristics of power battery pack at different rates of charge/discharge during fast charge/discharge are studied by numerical simulation. The heat dissipation effects of pure phase change material (PCM) cooling and liquid coupled with PCM cooling on the battery module are compared, and the control effects of the above two heat transfer methods on the temperature difference and maximum temperature are analyzed. The simulation results show that when 5C fast charging and 5C fast releasing, the optimal velocity of flow is 0.05m/s, the maximum temperature of the battery module is kept within 47.33 unmapped: inline-formula unmapped: math unmapped: msup unmapped: mrow unmapped: mrow unmapped: mo ∘C, and the temperature difference is 3.39 unmapped: inline-formula unmapped: math unmapped: msup unmapped: mrow unmapped: mrow unmapped: mo ∘C. Compared with the pure phase change cooling mode, the maximum temperature of the battery module is reduced by 34.57unmapped: inline-formula unmapped: math unmapped: msup unmapped: mrow unmapped: mrow unmapped: mo ∘ C, and the temperature difference is reduced by 1.14 unmapped: inline-formula unmapped: math unmapped: msup unmapped: mrow unmapped: mrow unmapped: mo ∘C. Therefore, the coupled system of liquid cooling and PCM has a good temperature control effect under the condition of fast charging and fast discharging.